Image quality control of analogue and digital radiographic imaging systems is in most cases performed by analyzing radiographic images made by exposing specific test objects, also called phantoms, to radiation under predefined exposure conditions.
These phantoms may comprise multiple specific test objects in order to enable verification of critical characteristics of a system.
The signal dependent noise, the dynamic range and the corresponding contrast resolution are characteristics of the system that set the image quality.
A single exposure of a test phantom positioned in the optical path between the source of radiation and the radiation detector is sufficient to enable measurement of these characteristics.
A contrast object absorbs part of the radiation to which it is exposed and generates, according to the principles of projection radiography and its semi-transparent nature, a residual image of the phantom on the detector.
By determining the thickness of the radiation attenuating object in each point the amount of attenuation of the radiation is controlled. Indirectly also the local radiation intensity in the shadow image which is generated on the detector is controlled.
Known embodiments of contrast phantoms comprise elongate (one dimensional) wedge-like test objects the thickness of which varies either continuously or step-wise from a minimal to a maximal value.
An example of such a phantom is the phantom denominated by the trade name DIGRAD marketed by the company PEHAMED. This phantom, used for quality control in digital radiography, comprises an elongate, step-wise, copper contrast wedge.
The EUROPHANTOM MAMMO (trade name) of the company PEHAMED which is used for quality control in the field of mammography comprises an aluminum step wedge phantom.
Other embodiments exists wherein the different parts of the contrast phantom are spread over the area of the contrast phantom (two-dimensional).
An example of such an embodiment is described in Proceedings of SPIE vol. 4320 (2001), p. 308-315.
The described contrast phantom is composed of a copper base platen to which on certain locations copper tablets are added and into which on other locations cut-aways are provided so that different thickness levels and a reference level are created.
In this way different locations on the surface of the radiographic detection system can be exposed to mutually very distinct exposure levels by means of a single exposure of the contrast phantom.
The measured signals and noise originating from different zones of analysis in the detected image are mutually compared. Deviations relative to reference values are evaluated for the purpose of periodic control of the intrinsic image quality of the entire radiographic system from source of radiation to image detection.
Irradiation originates from briefly applying by means of a high tension generator an adjustable high tension in between the cathode and the anode material (the target) of an X-ray tube.
The radiation originating in consequence of this—to which a radiographic detection system is exposed—is poly-chromatic and thus of a heterogeneous kind.
This bundle of photons consists of a large range of components of different wavelengths and corresponding energy levels.
For each field of application, for example mammography, general medical diagnostics, non-destructive material testing, radiotherapy and others a number of application dependent spectra have been defined in terms of anode material, level of applied high tension, type of material and thickness of intrinsic and external filters of radiation that are placed close to the source of radiation. These spectra are also called ‘radiation qualities’.
The attenuation of the radiation caused by an irradiated object is mainly caused by absorption of radiation depending on the local thickness of the irradiated material, the type of material the object is composed of and the spectrum of the incident radiation.
Given identical thicknesses, an object made of aluminum absorbs less radiation than a object made of copper.
The X-ray Mass Attenuation Coefficient which is characteristic for the absorbing properties of the material used, increases with augmenting atomic number.
In general this absorption coefficient decreases with increasing photon energy level.
As the energy level of the incident photons increases, the attenuation of the radiation by the object will decrease and the transmission of the object will increase.
This means that radiation of lower energy is relatively more attenuated than radiation of higher energy.
When compared with the incident radiation, not only the intensity of the residual radiation which results from penetration through the object, is attenuated but also the spectrum of the residual radiation is changed as a consequence of the non-uniform attenuation of low- and high energetic photons.
The relative contribution of soft, low-energetic components of the radiation has decreased relative to the contribution of harder, high energetic compounds of radiation.
This perturbation of the energy balance, known as the beam-hardening effect, becomes more explicit as the thickness of the object increases and as the incident radiation has a broader spectrum.
The radiographic system for which the image quality is to be assessed comprises a radiation integrating detector.
This detector is capable of locally measuring the amount of incident radiation and to convert this amount into a radiation image. Such a detector is for example a combination of a radiographic film and an intensifying screen, or a storage phosphor or a solid state radiation detector.
These detectors do not only integrate the incident radiation over time but also integrate the different energy levels that are present in the incident (residual) radiation.
Depending on the detector type used the spectral sensitivity for impinging radiation may vary.
Materials such as copper and aluminum have a Mass Attenuation Coefficient showing a continuous, strongly decreasing behavior with increasing photon energy for the entire spectrum of energies used within the medical diagnostic spectra.
If a contrast phantom with a substantially varying thickness is built from these materials or from materials with a similar behavior in order to obtain a large range of residual radiation intensities at the detector, problems can arise regarding the spectral sensitivity of the contrast object as well as regarding the usability of this contrast object for a wide range of radiation qualities.
Primo, mutual ratios of the different detected residual radiation signals with the reference signal, which corresponds with un-attenuated radiation, will strongly depend on the spectrum of the radiation used when exposing the contrast phantom.
This is caused by the fact that selection of a different energy quality results in the use of a higher or lower energy spectrum of the radiation.
Since the absorption coefficient of the used contrast object is highly sensitive to the selected energy level, large differences of the detected signal ratios will occur for different thickness steps of the wedge when strongly diverging energy qualities are used.
For example a copper step wedge with a thickness of 3.9 mm has the following behavior for different medical spectra used in general diagnostic radiography:
TABLE 1spectrumanodekVpfilterunatttenuated/residual ratioRQA5W742.5 + 21 Al300:1(100% ref.)RQA6W812.5 + 26 Al127:1(42%)RQA7W902.5 + 30 Al52:1(17%)RQA8W1002.5 + 34 Al27:1(9%)RQA9W1202.5 + 40 Al12:1(4%)
Since a minimal detected signal ratio between the un-attenuated irradiation spectrum and the most attenuated residual spectrum is required in order to obtain a meaningful image quality control, the use of a copper wedge is restricted to application in a very limited spectral range.
In addition component spread and early wear of the X-ray tube, a slightly erroneous setting of the tube voltage or the fact that the ripple of the generator voltage is too high may have as a consequence that a large portion of the anticipated tolerance margins on the nominal signal ratios are already consumed without there being any problem with the performance of the detection system itself.
For a contrast phantom made of copper the sensitivity of the signal detected under an absorption step with a thickness of 3.9 mm for a erroneously set tube voltage of +/−1 kV is
TABLE 2signal change/spectrumanodekVpeakfilter+/−1 kVpeak deltaRQA5W742.5 + 21 mm Al13.9%RQA6W812.5 + 26 mm Al9.1%RQA7W902.5 + 30 mm Al6.2%RQA8W1002.5 + 34 mm Al3.9%RQA9W1202.5 + 40 mm Al2.4%